An interpenetrating polymer network (IPN) is a composition of two incompatible polymers which exist in networks that are formed when at least one of the polymers is synthesized or cross-linked in the presence of the other. Systems in which both polymers are cross-linked are termed true-IPNs. Systems in which only one of the polymers is cross-linked are termed semi-IPNs. Both true- and semi-IPN systems may be referred to as thermosetting polymer networks.
The behavior of thermoplastics can be modified using semi-IPNs by forming thermoplastic semi-IPN systems. In such systems, the thermosetting polymer network is formed within a thermoplastic polymer. The coexisting structures are stabilized by physical cross-links in the thermoplastic phase.
In silicone-thermoplastic semi-IPNs, available from Huls America, Inc. of Piscataway, N.J. under the trademark Rimplast.RTM., the thermosetting network is formed by the addition of silicone oligomers. Such a system is formed as follows. First, the thermosetting component is formed by mixing a hydride- and a vinyl-functionalized silicone component. In a second step, the thermosetting component is compounded with a thermoplastic into strands in an extruder and subsequently pelletized. In a third step, the pellets are dryed and then cooled to ambient temperature. A platinum complex catalyst is sprayed on the pellets, typically with an inhibitor to prevent reaction. In the final step, the catalyst is activated causing cross-linking to occur while the pellets are injection molded or extruded into a final product. Silicon-thermoplastic semi-IPN systems are disclosed in U.S. Pat. Nos. 4,714,739 and 4,970,263. Other types of silicon-thermoplastic IPN systems are described in U.S. Pat. Nos. 4,500,688 and Re. 33,070. These references, and all other cited in this specification, are incorporated herein by reference.
Thermoplastics modified in this manner show good release characteristics, low wear and friction, increased dimensional stability and improved melt flow.
While prior art silicone-thermoplastic semi-IPN systems offer a number of advantages over the unmodified polymer, the impact strength of such modified thermoplastics may be lower than the unmodified thermoplastic. For example, the notched impact resistance of silicone-nylon 6,6 semi-IPN is about 10 percent lower than that of unmodified nylon 6,6 and the notched impact resistance of silicone-nylon 12 semi-IPN is as much as 70 percent less than unmodified nylon 12 when such modified polymers are formed according to the teachings of the prior art. See Arkles et al., "Polysiloxane-Thermoplastic Interpenetrating Polymer Networks," Adv. in Chem. Series No. 224--Silicon Based Polymer Science: A Comprehensive Resource, p. 181-199 (1990 Amer. Chem. Soc.).
While methods are known for improving the impact resistance of thermoplastic, such methods do not provide some of the other advantages of silicone-thermoplastic semi-IPNs. For example, Pape discloses that polymethylsiloxane fluids can significantly improve the Izod impact of polycarbonate. P. G. Pape, "Applications of Silicon-Based Chemicals in the Plastics Industry," CHEMSPEC USA 90 Symposium, Cherry Hill, N.J., October 1990. Such fluids, however, have a tendency to migrate, within the thermoplastic, which may affect properties of the thermoplastic. In other methods, rubber particles are grown and incorporated in thermoplastic; however, such methods are complex.
Thus, there is a need for a simple method to improve the impact resistance of thermoplastics while retaining the benefits of silicone-thermoplastic semi-IPN systems.